Sky & Telescope - May 2021 - 21

G R A ND SITE: CH A RLES TIM MER M A NS; HOW G R A ND
WORKS: G R A ND COLL A BOR ATION

tains) and Russia (east of the Ural range). " The Transantarctic
Mountains would also work, " he says, " although deploying
tens of thousands of radio antennas in Antarctica is going to
be expensive. " Currently, the full-scale version of GRAND is
budgeted at somewhere between $150 and $200 million - a
lot of money for a ground-based facility, but signiﬁcantly
cheaper than a space mission like POEMMA.
Timmermans and de Jong are very hopeful that GRAND
eventually will detect discrete sources of extremely highenergy neutrinos. If they do, it would be bad news for Anchordoqui's superwind model. If UHECRs are accelerated in galactic superwinds, as he believes, the acceleration takes place in
the tenuous halos of galaxies, where densities are probably
too low to produce large numbers of high-energy neutrinos.
" If GRAND ﬁnds discrete sources of high-energy neutrinos,
the superwind scenario cannot be correct, " Anchordoqui says,
" and UHECRs must be accelerated in the cores of galaxies. "
In that case, scientists will have to revisit the question of why
only some galaxies produce UHECRs.

But Anchordoqui isn't convinced that GRAND will have
a serious impact on revealing the sources of UHECRs. " My
personal opinion is that the radio technique is not yet mature
enough, " he says, " so I think AugerPrime is more likely to
make the big discovery. "
According to Timmermans, however, that won't happen
before 2025, and it will depend on how long it takes for the
individual detections to add up into clear, localized signals.
" If AugerPrime hasn't found discrete sources by 2030, " he
says, " I believe that GRAND will have the best chances of
achieving the breakthrough. "
Meanwhile, if the POEMMA mission gets the green light,
its observations may well yield an independent answer by the
mid-2030s. One way or the other, the elusive energetic particles probably won't stay a riddle for much longer.

Contributing Editor GOVERT SCHILLING is a science writer
who lives in The Netherlands. He has written dozens of books
and is currently writing a book on dark matter.

W GRAND FUTURE Far left: This windswept site in western China is the
home of the growing Giant Radio Array for Neutrino Detection. Standing at the site are Xiang-Ping Wu and Huang Yan (National Astronomical
Observatories of China). Near left: The setup for site survey operations.
Scientists used easily moved antennas to measure background noise.

m
Cos

ys
ic ra

Extensive
air showers

Radio emission

νμ

τ
ντ

10 km

νe

S GRAND The Giant Radio Array for Neutrino Detection will
eventually use thousands of radio antennas to detect the signal of cosmic rays and tau neutrinos. Unlike muon or electron
neutrinos, tau neutrinos will interact with atoms in neighboring
mountains to create a detectable particle shower. The antennas have a triple bow-tie design, with three perpendicular
arms. (Note that the mountain-facing slope the antennas are
deployed on will be gentler than shown here.)

s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 1

http://www.skyandtelescope.org

Sky & Telescope - May 2021

Table of Contents for the Digital Edition of Sky & Telescope - May 2021